1. Field of the Invention
The present invention relates to a chip scale marker and a marking method, and more particularly, to a method for marking characters on a wafer chip after calibrating a marking distance from a laser to the wafer, using a laser of a chip scale marker and a device thereof.
2. Description of the Related Art
In general, wafers used in a semiconductor manufacturing process are composed of several thousands to several tens of thousands of chips. It is required that characters or/and numbers are marked on a surface of each chip in order to classify the chips according to their production lot numbers. Presently, a chip scale marker using a laser beam is used as a tool for marking.
FIG. 1 is a schematic view of a general chip scale marker 10, illustrated with a wafer w. Referring to FIG. 1, the wafer w is placed on a wafer holder 20 and a laser system 30 is positioned below the wafer holder 20. A laser beam oscillated from a laser source of the laser system 30, is irradiated on chips of the wafer w via a galvano scanner (not shown) and an f-theta lens (not shown), and finally marks characters on the chips.
Above the wafer holder 20, a camera 40 is positioned for monitoring an object held by the wafer holder 20. The camera 40 is connected to an X-Y stage 50 and moves with the X-Y stage 50. The reference numeral 60 denotes a table, on which the X-Y stage 50 and the wafer holder 20 are placed.
FIG. 2 is a view showing a depth of focus D.O.F of a laser beam irradiated on the wafer. FIG. 3 is a view showing a warpage of wafer on a wafer holder. Referring to FIG. 2, a laser beam to be irradiated on a horizontal wafer chip from the f-theta lens 34 via the galvano scanner (not shown). Here, it is possible to obtain good quality of marking only if a marking surface is placed in the range of the depth of focus D.O.F. Here, the depth of focus is calculated as follows.
D.O.F=xc2x12xcex(ƒ/D)2
Here, D denotes the diameter of an incident beam, f denotes the focus distance and xcex is the wavelength of a laser beam.
However, in a wafer including a plurality of chips, there is a warpage in a certain direction due to weight of the wafer, coating on the wafer surface and other processes (refer to FIG. 3). This warpage becomes severer as the wafer is bigger, the wafer is thinner, and the wafer shrinks more, when the coatings on the wafer harden. If a height deviation h of wafer surface to be marked due to warpage is bigger than the depth of focus, the marking quality decreases because a beam laser density changes depending on the position of a chip on a wafer surface.
It is an object of the present invention to provide a method for measuring the wafer warpage, calibrating a marking distance, and marking a wafer.
It is another object of the present invention to provide a chip scale marker for the above marking method.
To accomplish the first object of the present invention, there is provided a method for marking, using a chip scale marker, wherein a laser beam is irradiated from a laser source on the wafer chips via a galvano scanner and an f-theta lens, the method comprising: (a) measuring position information of a plurality of points on the wafer; (b) transmitting the measured position information to a controller; (c) calculating a deviation between a marking distance between the f-theta lens and the point on the wafer surface and a focus distance of the f-theta lens from the transmitted position information; and (d) if the deviation is greater than a predetermined value in the step (c), calibrating the wafer chip to be positioned at the focus distance of the f-theta lens.
It is preferable that the step (a) is measured using a non-contact sensor, a laser sensor. Preferably, the step (a) is sequentially performed at each chip of the wafer or performed on a predetermined plurality of wafer chips positioned on at least one straight line crossing an axis of the wafer. It is preferable that the step (c) further comprises calculating a marking height deviation between the maximum and the minimum of the deviations. It is preferable that the step (d) comprises: (d1) equally dividing the marking height deviation into a predetermined number n, and forming the divided n regions at a marking surface of the wafer by forming contour lines with the equally divided height deviation; (d2) adjusting the wafer chips of a selected region at a predetermined distance from the f-theta lens; (d3) marking wafer chips of the selected region; and (d4) repeating steps (d2) and (d3).
The length of the equally divided height deviation may be smaller than a depth of focus of the f-theta lens, preferably is smaller than xc2xd of the depth of focus of the f-theta lens.
To accomplish another object of the present invention, there is provided a chip scale marker that includes a laser system for marking a wafer, a wafer holder for supporting a wafer to be processed, and a camera which moves while being connected to an X-Y stage over the wafer holder and monitors an object held by the wafer holder, the chip scale marker further comprising: a sensor for measuring a vertical position of each chip of the wafer; and a means for moving the wafer holder in a vertical direction. It is preferable that the sensor is connected to the X-Y stage.
To accomplish the second object of the present invention, there is provided a chip scale marker, further comprising a means for moving the laser system for wafer marking in a vertical direction, instead of a means for vertical moving of the wafer holder.
In addition, to achieve the second object of the present invention, there is provided a chip scale marker, further comprising; a focus distance correction lens located between the laser oscillator and the galvano scanner; and a means for moving the focus distance correction lens in a horizontal direction.